Compared with conventional polyacrylic acid-based thickeners, nonionic polyurethane-based thickeners that exhibit thickening properties upon aggregation of hydrophobic groups are recently attracting more attention as thickeners, particularly for products such as cosmetics.
These polyurethane-based thickeners exhibit more increased thickening properties through the association of hydrophobic groups located at the terminals. Because of their nonionic nature, these polyurethane-based thickeners are excellent in salt tolerance, and can suitably be used in combination with ionic surfactants, while being capable of imparting additional rheological properties.
It is known in this field of technology that the thickening properties increase along with an increase in the chain length of hydrophobic groups located at the terminals (Non-Patent Document 1).
Examples of known polyurethane-based thickeners include: those having three hydrophobic groups that are linked through hydrophilic polyester segments of at least 1500 molecular weight each with at least two of the three hydrophobic groups being located at the terminals (Patent Document 1); and those obtained by the reaction of (a) at least one water-soluble polyetherpolyol, (b) at least one water-insoluble organic polyisocyanate, (c) at least one monofunctional hydrophobic organic compound selected from the group consisting of compounds containing a hydrogen atom reactive with an isocyanate and organic monoisocyanates, and (d) at least one polyhydric alcohol or polyhydric alcohol ether (Patent Documents 2 and 3).
Also known are a polyurethane-based thickener characterized by comprising a monohydric alcohol containing at least one other polar group and a trihydric alcohol as a polyhydric alcohol (Patent Document 4) and a viscosity modifier obtained by using a polyether monohydric alcohol containing as a hydrophobic group at least one branched chain or at least one secondary hydrocarbon group (Patent Document 5).
As described in the Non-patent Document, as the number of carbon atoms at the terminal alcohol increases, the thickening properties of the polyurethane-based thickeners obtained by such conventional techniques become greater, due to the increased association power of the hydrophobic groups. However, an increased number of carbon atoms at the terminal alcohol leads to poor affinity for water, and aqueous solutions containing such thickeners are likely to become tacky. For example, when such thickeners are added to cosmetics as a thickening additive, the cosmetics exhibit a heavy feel when used. Another drawback is that because molecular chain terminals modified into hydrophobic groups lower the affinity for water, the aqueous solution becomes turbid. To avoid turbidity, the proportion of hydrophobic groups may be decreased so that the transparency of the aqueous solution is improved. In this case, however, the viscosity decreases when a cationic surfactant (those used particularly for hair care products) is added.
Polyurethane-based thickeners are nonionic. Thus, unlike polyacrylic acid-based thickeners, polyurethane-based thickeners, when used in combination with a cationic surfactant, do not generally produce the precipitation of aggregates resulting from the interaction between cations and anions. However, a polyurethane-based thicker in combination with a cationic surfactant results in a thickened liquid with poor transparency, thereby giving the undesirable appearance.